1. Field of the Invention
The present invention relates to a crystal oscillator circuit that uses CMOSFETs (Complementary Metal Oxide Semiconductor Field Effect Transistors). The invention particularly relates to a structure of a crystal oscillator circuit in which an amplifying section consisting of CMOSFETs excites a resonating section containing capacitors and a crystal.
2. Description of the Related Art
In recent years, along the digitalization of electronic devices, there has been progressive improvement in crystal oscillator circuits that can produce accurate frequencies. Crystal oscillator circuits are being used as frequency signal (clock pulse) generator circuits in computers, as well as in electronic timepieces.
A conventional crystal oscillator circuit has been constructed of an oscillator circuit having an amplifying section and a resonating section, and a constant-voltage circuit. The constant-voltage circuit lowers the source voltage of a battery or the like to a regulated voltage, and this voltage is used as the power source of the crystal oscillator circuit. The amplifying section is constructed of CMOSFETs, and a high-resistance feedback resistor is connected between an input terminal and an output terminal of the amplifying section.
The resonating section is for obtaining a predetermined frequency, and has an oscillator circuit consisting of two capacitors and a crystal. This crystal is connected between the input terminal and the output terminal of the amplifying section. The crystal in the resonating section mechanically oscillates, and the amplitude of this oscillation gradually reduces. Therefore, the amplifying section amplifies the oscillation of the crystal oscillator, and applies positive feedback between the resonating section and the amplifying section, thereby compensating the allenuation maintaining the oscillation of the crystal oscillator.
In the case of a portable electronic device such as an electronic timepiece, a silver oxide battery or a lithium battery having a voltage range of about 1.3 V to 3.0 V is used for the power source of this device. In order to decrease the number of times of replacing a battery of the portable device, the voltage of the oscillator circuit or a divider that is connected to a latter stage of this oscillator circuit is lowered from the battery voltage by a constant-voltage circuit. The driving power of the device is restricted in this way.
However, when the electronic timepiece is completely in the stopped status, or when the timepiece has stopped temporarily due to some disturbance in the normal driving status, an attempt to drive the oscillator circuit with the constant-voltage circuit at the oscillation start initial stage brings about such a problem that the oscillation starting period of the oscillator circuit becomes long or the oscillator circuit does not start oscillating.
This problem occurs because the DC bias of the amplifying section is always fixed to one half of the source voltage, as the high-resistance feedback resistor is connected between the input terminal and the output terminal of the amplifying section of the crystal oscillator circuit. Therefore, the oscillator circuit cannot operate unless a voltage of about two times the threshold voltage of the CMOSFETs, that constitute the amplifying section, is supplied as the source of the oscillator circuit, which is disadvantageous for driving at a low voltage.
The phenomenon that the starting period of the oscillator circuit is long at the oscillation start initial stage and the phenomenon that the oscillator circuit does not oscillate are attributable to a fact that amplification factors of an amplification PMOSFET (P-channel MOSFET) and an amplification NMOSFET (N-channel MOSFET), that constitute the CMOSFETs, are small.
Therefore, when the oscillator circuit stops oscillation, a measure may be taken to increase the amplification factors of the amplification PMOSFET and the amplification NMOSFET. According to this measure, the oscillation status and the non-oscillation status of the oscillator circuit are detected. When the oscillation is stopped, the oscillator circuit is driven at the source voltage in this status. When the oscillator circuit is in steady oscillation, the constant-voltage circuit lowers the source voltage to the regulated voltage in order to restrict power consumption, and the oscillator circuit is driven at this lower voltage.
However, according to the conventional oscillator circuit, the DC bias point is determined as one half of the source voltage. Therefore, for the oscillator circuit to operate stably, the source voltage for application to the oscillator circuit cannot be set equal to or lower than the threshold voltages of the amplification PMOSFET and the amplification NMOSFET. Consequently, there has been a limit to making the oscillator circuit oscillate at low power.
Further, when the oscillator circuit is driven at the source voltage during the oscillation starting period and also when the source voltage is lowered to the regulated voltage with the constant-voltage circuit during the steady oscillation period, there are the following problems. The regulated output voltage varies due to a shortage in responsiveness to the frequency of the constant-voltage circuit during the voltage fall period from when the oscillator circuit starts oscillation till when the oscillation becomes steady. As a result, an inconvenience occurs in that the oscillator circuit stops oscillating or the oscillation is not stabilized.
In order to increase the responsiveness to the frequency of the constant-voltage circuit, the frequency characteristics may be improved by increasing the drive current of the constant-voltage circuit. However, this leads to an increase in total power consumption of the crystal oscillator circuit, which is disadvantageous for the driving at low power.
Further, in general, a PMOSFET and an NMOSFET that constitute an integrated circuit have processing size errors due to variations in the environment temperature and manufacturing. As the regulated voltage generated by the constant-voltage circuit and the characteristics of the amplifying section vary due to the processing size errors, the oscillator circuit cannot provide stable oscillation characteristics.
It is an object of the present invention to provide a crystal oscillator circuit that can minimize variations in characteristics during a period from when the oscillator circuit starts oscillating until the oscillation becomes steady, thereby to stabilize the oscillation of the oscillator circuit, in the oscillator circuit consisting of a resonating section having capacitors and a crystal oscillator, and an amplifying section that excites this resonating section.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a crystal oscillator circuit using a CMOSFET, the crystal oscillator circuit comprising: an oscillator circuit that is constructed of a resonating section having capacitors and a crystal oscillator, and an amplifying section having a CMOSFET for exciting the resonating section; a reference current source circuit for generating a reference current which determines a bias current of the amplifying section that has a reference current control section for setting a reference current and a reference current generator section for generating a reference current; and a control signal generator circuit that detects whether the oscillator circuit is in the oscillation status or in the non-oscillation status, and when the oscillator circuit is in the non-oscillation status, that generates a control signal for controlling the reference current control section to set the reference current which makes a bias current of said amplifying section larger than that in the oscillation status.
According to a second aspect of the invention, there is provided a crystal oscillator circuit of the above first aspect, wherein the oscillator circuit, the reference current source circuit, and the control signal generator circuit are connected between a reference potential and the other end of a battery of which one electrode is connected to the reference potential, respectively, and the crystal oscillator circuit operates using the battery voltage as the source voltage.
According to a third aspect of the invention, there is provided a crystal oscillator circuit of the above second aspect, wherein the reference current control circuit consists of at least two reference resistors connected in parallel between the reference potential and a connection point that is connected to the reference current generator circuit, and the crystal oscillator circuit changes over a value of a combined resistance of the reference resistors based on a control signal from the control signal generator circuit.
According to a fourth aspect of the invention, there is provided a crystal oscillator circuit of the above third aspect, wherein the crystal oscillator circuit changes over the combined resistance based on the control signal to at least one switching transistor that is connected in series with the reference resistors.
According to a fifth aspect of the invention, there is provided a crystal oscillator circuit of the above fourth aspect, wherein the switching transistor is a control MOSFET.
According to a sixth aspect of the invention, there is provided a crystal oscillator circuit of the above first aspect, wherein a constant-voltage circuit for generating a regulated voltage that is a reduced voltage of the source voltage of the battery is provided between a reference potential and the other end of a battery of which one electrode is connected to the reference potential, and the oscillator circuit, the reference current source circuit, and the control signal generator circuit are connected between the reference potential and a line for supplying this regulated voltage, respectively, and the crystal oscillator circuit operates using the regulated voltage as the source voltage.
According to a seventh aspect of the invention, there is provided a crystal oscillator circuit of the above sixth aspect, wherein a current control device is connected between the amplifying section and the line, and the current control device is controlled by the reference current source circuit.
According to an eighth aspect of the invention, there is provided a crystal oscillator circuit of the above seventh aspect, wherein the current control device is a PMOSFET, a gate of the PMOSFET is connected to the reference current source circuit via a high-resistance resistor, and a voltage applied to the gate is linked to a change in the bias current to the amplifying section.
According to a ninth aspect of the invention, there is provided a crystal oscillator circuit of any one of the above first to eighth aspects, wherein the control signal generator circuit has two charging and discharging circuits connected in parallel that have mutually different charge and discharge statuses for the same input signal, output signals of the two charging and discharging circuits are input to an OR circuit, and an output of the OR circuit becomes the control signal.
In the crystal oscillator circuit of the present invention, there are provided a reference current source circuit that has a reference current generator section that generates a reference current for determining a bias current of an amplifying section, and a reference current control section that determines the reference current by controlling the reference current generator section; and a control signal generator circuit that controls the-reference current control section.
In the structure of the present invention, the control signal generator circuit detects oscillation and non-oscillation of the oscillator section. The reference current control section is controlled based on an output signal of a result of the detection, thereby to set a current that flows to the reference current source circuit.
The reference current source circuit controls the bias current of the amplifying section during the oscillation period and during the non-oscillation period, and sets amplification factors of the amplifying section that are suitable for the oscillation starting period and the steady oscillation period.
As a result, it is possible to minimize variations in characteristics during the period from when the oscillator section starts oscillation until the oscillation becomes steady, and make the section oscillate instantly with low power. Further, it is possible to obtain oscillation characteristics that make it possible to achieve a stable and steady oscillation.